1. Field of the Invention
The present invention relates to panel displays, and in particular to pixel driving circuits.
2. Description of the Related Art
In a pixel of an organic light-emitting diode (OLED) display, charges are stored in a storage capacitor for controlling the luminance of an OLED via a thin-film transistor (TFT). Referring to FIG. 1, a schematic diagram of a conventional pixel circuit is shown. The pixel circuit 100 includes an N-type TFT 102, a storage capacitor 104 and an OLED 106. The two ends of the storage capacitor 104 are respectively coupled to the gate G and the source S of the TFT 102. The voltage drop of the storage capacitor 104 is denoted by Vgs. The positive end of the OLED 106 is coupled to the source S of the TFT 102, whose voltage level is denoted by VOLED. The current flowing by the TFT 102 is controlled by the voltage drop Vgs, with the current IOLED of the OLED 106 being equal to K*(Vgs−VTH).sup.2. The voltage drop Vgs is the voltage difference between the pixel voltage Vdata and the voltage level VOLED at the positive end of the OLED 106. Therefore, the luminance of the OLED 106 can be controlled by adjusting the pixel voltage Vdata.
However, when the above-mentioned TFT 102 is operating, a shift of the threshold voltage occurs on the TFT 102. The amount of voltage shift is related to the manufacturing process, operation time, and the current of the TFT 102. Therefore, in terms of all pixels on the display panel, due to the difference of the pixels in the operation time, conductive current, and manufacturing process, the amount of shift of the threshold voltage of each pixel is different, which in turn causes the luminance and the received pixel voltage of each pixel to have a different corresponding relationship. Therefore, the issue of non-uniform frame luminance occurs.
In addition, the OLED 106 has an increasing voltage drop, which is an increasing VOLED, along with the usage time. Referring to FIG. 2, a characteristic diagram of the OLED 106 is shown. From FIG. 2, it can be seen that the OLED 106 has an increasing VOLED under a long usage time. Therefore, the conductive current IOLED is reduced under the long usage time according to the equation Vgs=Vdata−VOLED. The decreasing current IOLED causes the pixel voltage Vdata to be unable to drive the OLED 106 to reach the predetermined luminance. Thus the overall luminance of the display frame is reduced.
There is therefore a need for a pixel driving circuit and a pixel driving method thereof to solve the variation of the thin-film transistors (TFT) and the aging of the OLED 106.